Dual fuel systems are well known. Such systems typically consist of a means for selecting the fuel to be utilized, a liquified-to-gaseous fuel converter or vaporizer, and a mixer for gaseous fuel and air.
Typically, such systems utilize electrically operated solenoid valves for the alternative selection of fuels, e.g., one valve in the liquid propane line between the propane storage tank and the inlet to the converter or vaporizer, and another in the gasoline line supplying the carburetor. These solenoid valves are typically activated by a switch mounted for convenient operation by the vehicle driver. However, they frequently do not work well in the environment of the engine, i.e., it is difficult to locate a reliable voltage source in the vehicle's electrical system which is the right voltage, which is available during engine starting and which does not adversely affect the engine ignition system. In addition to the electrical lock-offs for each type of fuel, a vacuum or oil pressure safety switch is generally required. In one aspect, the present invention obviates electrical problems of this type by utilizing a single mechanically operated valve structure with a vacuum lock-off valve in the converter.
Liquified petroleum gas, propane and the other "gaseous" fuels are normally held in a tank under sufficient pressure and at a temperature to remain in a liquid state, and are herein referred to as "liquified gas" fuels. Conversion of such liquified fuel to a gaseous state at a suitable pressure and temperature is accomplished in the converter or vaporizer.
Known converters are generally of the two stage variety. The first stage generally includes a small diameter first stage diaphragm within a chamber warmed by hot engine coolant to actuate a normally open liquified gas inlet valve, and the second stage generally includes a large diameter diaphragm to actuate a normally closed valve responsive to engine demand to supply fuel. However, the converter in some systems (such as disclosed in U.S. Pat. No. 3,528,787 dated Sept. 15, 1970 and assigned to the assignee of this application) utilized a single diaphragm responsive to engine demand but operable at a positive pressure to supply gaseous fuel. Such system continued to use a solenoid operated fuel lock-off valve. In addition, the operating pressure to which such systems were regulated was not variable in response to engine demand.
In another aspect, the converter of the present invention provides a positive "lock off" of gaseous fuel to the converter, combines the two valve functions in a single structure and utilizes two diaphragms of equal size thereby realizing ecomony of manufacture and increased interchangeability of parts. In addition, the utilization of a large fuel inlet valve diaphragm results in very quick starting capability because relatively little drop in intake manifold pressure is required to activate the inlet valve. This reduces the problems common with the low engine starting speeds and the resultant small drop in manifold pressure normally experienced during cold weather conditions.
In another aspect, the converter of the present invention modulates the positive pressure at which gaseous fuel is applied to the mixer as a function of engine demand. A simple mechanical adjustment of the modulator then provides a power adjustment for different fuels.
In the "gasoline" mode, the converter vacuum line is opened to the atmosphere and the consequent absence of pressure differential on the liquified fuel inlet diaphragm of the converter effects a positive lock-off of the liquified fuel inlet valve permitting the conventional carburetor to function in a normal fashion, i.e. with air passing through the air filter and mixer into the carburetor.
In the "liquified gas" mode, the gasoline valve is "off" and engine vacuum is connected to the converter. When the engine is turned over or "cranked", engine vacuum reduces the pressure on one side of the liquified fuel inlet valve diaphragm to permit liquified fuel to enter a fuel passage in the converter. The liquified fuel is admitted responsively to engine demand through a second valve into a gaseous fuel chamber where it expands to a gas, normally regulated to 11 inches w.c. but modulated by engine vacuum.
Fuel economy is enhanced in the present invention by elevation of the temperature of the liquified gas to encourage vaporization in the gaseous fuel chamber where fins are warmed by heat from engine coolant piped through the body of the converter. At higher speed operation, the elevation in temperature is less due to the reduced time of passage of the fuel through the converter. Thus, the engine will benificially run slightly richer at high speeds. Conversely, greater elevation in the temperature of the fuel is achieved during cruise conditions, thereby increasing its volume and resulting in a leaner fuel mixture. The temperature of the gas exiting known prior art converters generally does not vary significantly as a function of engine speed, although this feature may be found to some limited extent in the Hallberg patent referenced above.
Additionally, engine vacuum may be used to modulate the response of the second diaphragm to fuel chamber pressure. For example, engine vacuum during cruise condtions, which is between 7 inches h.g. and 20 inches h.g., may be used to reduce the pressure in the gaseous fuel chamber from approximately 11 inches w.c. to approximately 8 inches w.c. to insure an economically lean fuel mixture during cruise conditions.
Early mixers consisted of a tube in the existing gasoline carburetor at a point where the venturi narrows. However a venturi is not very effective as a metering device when air flow is low and/or slow and typically has a narrow control range unsuitable for both low speed and high speed engine requirements. Subsequent mixer designs have utilized a tapered plug device in the opening of the carburetor, with the plug moving up and down in response to variations in air flow. Such "variable venturi" or "air valve" carburetors have not generally been satisfactory due to hysteresis, i.e., weight of the plug, and the short distance of plug travel, i.e., typically approximately 15.9 mm.
Improved mixer structures such as disclosed in the Hallberg U.S. Pat. No. 3,528,787 employ a plunger structure comprising a large piston responsive to venturi vacuum, a small piston which variably obstructs air flow, and a needle positionable in a fuel orifice on the opposite side of the air passage from the large piston. However, such systems have a relatively large mass and generally exhibit hysteresis problems and relatively poor fuel economy.
In other systems, a diaphragm responsive to engine vacuum is used to meter fuel. However, the movement of the diaphragm is relatively small, and it is difficult to maintain adequate lock-off pressure and acceptable control with movement over such a small distance, thereby necessitating great manufacturing precision.
In another aspect, the present invention provides a more compact mixer with reduced hysteresis by locating the fuel valve within the air valve structure. In one embodiment, the plunger structure includes a relatively large round piston which provides a larger effective surface area than a diaphragm of the same diameter because the ratio of piston surface areas subjected to pressure differentials is greater than the same ratio on similar diaphragm type mixers. In a second embodiment, the plunger structure includes a smaller round piston with a reduced mass generally flat bladed portion as the air valve. In both embodiments, the piston is actuated by the venturi vacuum which results from air flow, and the fuel valve (either needle valve or slotted tube) is located within the air valve structure. Actuation of the piston in response to venturi vacuum opens the fuel valve to supply a fuel and air mixture to the engine through the conventional carburetor and results in stable piston positions and uniform fuel flow at various engine speeds.
The location of the fuel valve within the air valve structure permits the use of a very long needle or slotted tube and thus enhances fuel control. In addition, the fuel transfer tube may be utilized as a guide for the plunger/piston structure, obviating the need for a separate guide within the cylinder in which the piston travels and reducing valve wear problems. This also facilitates the use of labyrinth seals on the piston. The effect of hysteresis is also substantially lessened and may become negligable by comparison to the relatively long piston travel.
In one embodiment of the mixer, air flow is regulated by the distance between one face of a large round piston and an annular surface coaxial with the piston. Since the area of the "cross section" of the air passage in this embodiment is effectively the product of the diameter of the annular surface multiplied by the distance of the piston from the surface, a relatively large volume of air is admitted for a given increment of piston travel, and total piston travel can be relatively short resulting in a compact mixer.
The use of a generally flat air valve plunger structure has been found advantageous because the ratio, of air control surface area to the surface area of the piston rendered ineffective by the presence of the plunger, is greater than is possible with a round plunger. The ratio of the cross-section area of the air passage to the area on the face of the piston diameter is relatively large, thus permitting a compact mixer.
In another aspect of the invention, the use of a positive pressure eliminates the need for mechanical or electrical priming and represents a significant advantage, particularly at low temperatures.